The long term primary objective and continuing theme is an understaznding of the regulation by and of the procoagulant enzyme thrombin. to apporach the goal, specific aims will focus on studies of teh specificity of the enzyme and the ways that specificity is regulated, the activation by thrombin of an anticoagulant system based on the vitamin K-dependent zymogen, protein C, and the fibrinolytic system, especially from the viewpoint of how thrombin and protein C might contribute to the regulation of fibrinolysis. The expectations of a regulatory system based on these proteins are derived in part from dramatic thrombotic disease that has been associated with derangements of individual components, and in part from a perception that complex biochemical systems require complex regulation. Among the tools to be used to address the aims are: derivatives of thrombin, including a genetically abnormal enzyme, that have characteristic pertubations of specificity; thrombomodulin, and endothelial cell surface cofactor that binds thrombin with high affinity and reversibility changes the substrate specificity of the enzyme; protein C, a vitamin K-dependent enzyme which is activated, by thrombin vound to thrombomodulin, to yield a potent and specific anticoagulant; assays of the fibrinolytic system, including whole animal and organ perfusion systems to study factors that elicit activators of fibrinolysis from extravascular sources. Each of these tools will be applied to purified components, to the interface between solutions and the vascular surface, and in intact animals.